/Engine/Plugins/Runtime/GeometryProcessing/Source/DynamicMesh/Public/MeshRefinerBase.h
Check if edge collapse will create a face-normal flip. Also checks if collapse would violate link condition, since we are iterating over one-ring anyway. This only checks one-ring of vid, so you have to call it twice, with vid and vother reversed, to check both one-rings
bool CheckIfCollapseCreatesFlipOrInvalid ( int vid, int vother, const FVector3d & newv, int tc, int td ) const